This invention relates to linear communicationssystems which use optical coupler circuitry and, in particular, to optically coupled circuitry which has sufficient linearity and temperature insensitivity to be used in a telephone system.
Various attempts have been made to eliminate transformers with the use of solid-state electronics. One solid-state combination suggested is the light emitting diode (LED) - light detecting diode (LDD) pair. One serious disadvantage of an LED-LDD pair optical coupler is the inherent non-linear transfer characteristics and the poor temperature sensitivity. Various attempts have been made to compensate for these undesirable characteristics of the LED-LEDD pair to allow the use thereof in analog systems which require a relatively high degree of linearity (low distortion) and long term stability.
One such attempted solution is the use of two substantially identical serially connected LEDs connected to the output of an amplifier. Two LDDs are physically located with respect to the LEDs such that light from the first LED impinges on the first LDD, and light from the second LED impinges on the second LDD. The first LDD is coupled to the input of the operational amplifier so as to create a negative feedback path. The second LDD is coupled to output circuitry. The negative feedback path serves to help linearize the output signal with respect to an input signal. One serious problem associated with this configuration is that the two serially connected LEDs must have essentially identical electro-optical characteristics in order to obtain reasonably good linearity. The requirement of matching the characteristics of two LEDs closely makes such an optical coupler economically unattractive.
One solution to the above problem of matching diodes is the use of two parallel LEDs with separate series resistors coupled to each. The LEDs are matched electrically and optically as closely as possible. Differences in the electrical and optical characteristics can be somewhat attenuated at any one operating point by varying one or the other of the resistors to vary the current through either LED. A major disadvantage of this configuration is that while one setting of resistor values may produce a high-quality linear response at one operating point, the same resistor values will result in unsatisfactory response at other operating points.
It would be desirable to have a solid-state optical coupler which does not require very close matching of the electro-optical characteristics of components but does provide relatively high linearity and is relatively insensitive to temperature variations.